Scattered cells and lymphoid
tissue, as well as slightly more organized lymph nodules, are
commonly found in the mucosa of the GI tract. Lymph nodules in
the ileum are referred to as
Peyer's patches
. Again, examine the
lymphoid cells
(mostly lymphocytes and plasma
cells).

Lymph nodes are more organized
organs than are nodules. They have a
capsule
,
subcapsular sinus
,
hilus
, supporting connective tissue
trabeculae
, and
trabecular sinuses
. Arteries, veins, and efferent
lymphatics are all located at the hilus. Afferent lymphatics
enter the capsule. Lymph nodes often appear "airy"
because of their system of lymph spaces or sinuses. Note the
outer cortex
containing lymph nodules and its transition to the
inner medulla
. In the medulla, the lymphoid
tissue is organized as irregular cords of cells,
medullary cords
, surrounded by
medullary sinuses
. Note the ingrowth of trabeculae
from the capsule into the node. The node is composed of many
free cells
,
mostly lymphocytes and plasma cells. Also note the distribution
and density of
reticular
cells
in the lymph node.

Note the dense connective tissue
capsule
covered with mesothelium (peritoneum). Heavy connective tissue
trabeculae
extend into the organ from the capsule to provide support. The
trabeculae also serve to carry blood vessels to/from the vascular
spaces in the spleen. Examine the two major components of the
spleen the
white
pulp
and
red
pulp
. The white pulp can be organized as
splenic (lymph) nodules
with the characteristic
central artery
(arteriole) in each nodule. The
germinal centers
of the nodules are mainly
B cells. In addition, the lymphatic tissue not organized as nodules
can be seen as the
periarterial
lymphatic sheath
(a thin cuff of tissue made up of T lymphocytes
that surrounds the central arteries). Both red and white pulp
are supported by reticular fibers that form a supporting framework
of the spleen.

Note that the thymus consists
of an outer
capsule
,
a darker stained
cortex
,
and a lighter central
medulla
. The cortex is darker because of the
high concentration of small lymphocytes. Smaller subdivisions
of the gland are called
lobules
.
Interlobular septa
separate the lobules from
each other.
Intralobular
trabeculae
extend from the capsule into the cortex. Within
the medulla are conspicuous eosinophilic, whorled structures
called
Hassall's
corpuscles
composed of a concentric mass of epithelioreticular
cells joined together by many desmosomes. These are diagnostic
of the thymus. Among the numerous small lymphocytes are many
thymic reticular
cells
(epithelial origin). Note particularly that the thymus
lacks lymph nodules and sinuses. Recall that after puberty the
thymus undergoes involution and is infiltrated by fat as lymphocytes
degenerate (the organ is, however, still functional)

Note in the tonsil that the lymphoid
tissue is arranged as
nodules
(with germinal centers) and diffuse
accumulations of lymphocytes. Try to identify the
rudimentary capsule
. The free surface is covered
with
stratified
squamous epithelium
(continuous with the oral cavity) which
dips into the organ forming epithelial invaginations called
tonsillar crypts
.
Note that many lymphocytes (very small darkly stained, round
nuclei) migrate through the epithelium and nearly obscure its
identity.